The mutational expansion of trinucleotide repeat sequences has been identified as a mechanism underlying the transmission of a variety of heritable disorders (e.g. fragile X syndrome, and Huntington's disease). These diseases are characterized by anticipation, an inheritance pattern in which successive generations are afflicted with greater severity and/or earlier onset. These patterns are explained by the intergenerational expansion of trinucleotide microsatellite sequence embedded in specific genes. The expansions are presumed to occur during early gametogenesis, or early embryogenesis, leading to the eventual alteration of gene expression and/or protein function in somatic tissues. We asked if repeat expansion occur in neoplastic cells, as a mechanism through which to alter cancer genes. In preliminary work we focused on testicular cell cancer, the familiar form of which is characterized by the anticipation inheritance pattern. Using the Repeat Expansion Detection (RED) technique we observed a high frequency of long (CAG)n triplet repeat tracts in testicular cell lines and in members of testis cancer families. Subsequent immunologic analyses of testis tumor cell lines have revealed the expression of proteins with expanded polyglutamine tracts, which are associated with some repeat expansion disease phenotype. In the proposed work, we will perform studies to address the hypothesis that an expanded (CAG)n tract(s) defines a locus residing in or near a gene important to testicular tumorigenesis. Our specific aims are: 1)To clone and sequence the locus containing the expanded (CAG)n tract. 2) To determine the chromosomal location of the locus containing the expanded (CAG)n tract. 3) To perform mutational analysis on the identified locus in an extended series of sporadic and familial testicular tumors for (CAG)n expansions and other alterations. 4) To study the expression of the identified locus.